Ultrafast processes and excited state dynamics in conjugated polymers

Abstract

The photophysics of a number of conjugated polymers used in organic display applications has been studied using ultrafast techniques. An updated method for measuring the intersystem crossing rate was developed. This has been used to measure the intersystem crossing in a number of conjugated polymers and oligomers. It was found that as expected the intersystem crossing in conjugated polymers is generally very low. The intersystem crossing in a molecule that exhibits dual fluorescence depending on the polarity of the environment was investigated. In polar solvents when the molecule forms a twisted interconnected charge transfer state the intersystem crossing rate was found to be ten times that of the planer locally excited state found in non-polar media. The nature of charge generation in conjugated polymers has also been investigated, it was found that in the prototypical conjugated polymer, polyfluorene, there is no intrinsic charge photogeneration. However, charges can be formed by singlet-singlet annihilation reactions. This is made very efficient in the solid state by the rapid diffusion and migration of excitons. The photophysics of a novel polyfluorene, polyspirobifluorene, was investigated thoroughly; it has been found that the addition of the spiro side group containing electron-donating groups causes a charge transfer state to form between the backbone and the side group. This can act as a reservoir for the emissive singlets, thus giving the polymer a long tail in the fluorescence decay. Finally, ultrafast pump probe spectroscopy was used to investigate on chain migration of excitons in dilute solutions of a ladder type polymer. The migration to the lowest energy segment of the chain shows up as a shift to the red of the photobleaching signal in the pump probe spectrum. This shows that the low energy states become occupied after the initial excitation, as the high energy states become depopulated by the migration, this process taking about 6.2ps.